MAINTAINERS: update brcm dts pattern

Commit 8c0b9ee8 ("MIPS: Move device-trees into vendor
sub-directories") moved the files, update the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>